1. Field of Invention
The present invention relates generally to a hybrid electric vehicle (HEV), and specifically to a strategy to start an engine in an HEV with minimal torque disturbance to the powertrain.
2. Discussion of the Prior Art
The need to reduce fossil fuel consumption and pollutants from automobiles and other vehicles powered by internal combustion engines (ICEs) is well known. Vehicles powered by electric motors have attempted to address these needs. However, electric vehicles have limited range and limited power coupled with the substantial time needed to recharge their batteries. An alternative solution is to combine both an ICE and electric traction motor into one vehicle. Such vehicles are typically called hybrid electric vehicles (HEV""s). See generally, U.S. Pat. No. 5,343,970 to Severinsky.
The HEV has been described in a variety of configurations. Some HEV patents disclose systems where an operator is required to select between electric and internal combustion operation. In other configurations the electric motor drives one set of wheels and the ICE drives a different set.
Other, more useful, configurations have developed. A series hybrid electric vehicle (SHEV) is a vehicle with an engine (most typically an ICE), which powers a generator. The generator, in turn, provides electricity for a battery and motor coupled to the drive wheels of the vehicle. There is no mechanical connection between the engine and the drive wheels. A parallel hybrid electrical vehicle (PHEV) is a vehicle with an engine (most typically an ICE), battery, and electric motor combined to provide torque to power the wheels of the vehicle.
A parallel/series hybrid electric vehicle (PSHEV) has characteristics of both the PHEV and the SHEV. The PSHEV is also known as a torque (or power) splitting powertrain configuration. Here, the torque output of the engine is given in part to the drive wheels and in part to an electrical generator. The generator powers a battery and motor that also provide torque output. In this configuration, torque output can come from either source or both simultaneously. The vehicle braking system can even deliver torque to drive the generator to produce charge to the battery (regenerative braking).
The desirability of combining the ICE with an electric motor is clear. The ICE""s fuel consumption and pollutants are reduced with no appreciable loss of performance or vehicle range. A major benefit of parallel HEV configurations is that the engine can be turned off during periods of low or no power demand from the driver (e.g., waiting for a traffic light). This improves fuel economy by eliminating wasted fuel used during idle conditions. The motor can then propel the vehicle under conditions of low power demand. In some configurations, the engine can be disconnected from the motor and powertrain when it is not running by opening a disconnect clutch. As power demand increases, the engine can be restarted and reconnected to provide the requested torque.
Developing a strategy to start an HEV engine and transfer primary torque production of the powertrain from the motor to the engine with minimal torque disturbance is needed for successful implementation of a parallel HEV. If the engine is disconnected from the powertrain, starting the engine would involve maintaining the vehicle""s response to the driver""s demand using the motor while simultaneously closing a clutch that connects the engine to the powertrain (disconnect clutch) and start running the engine. Torque supply to the powertrain should be transferred from the motor to the engine smoothly in order to avoid any disturbance to the driver.
Strategies to turn on an HEV""s engine are known in the prior art. See generally, U.S. Pat. No. 6,054,776 to Sumi; U.S. Pat. No. 6,026,921 to Aoyama et al.; and U.S. Pat. No. 5,865,263 to Yamaguchi, et al. Unfortunately, no strategy is known to start a parallel HEV engine while maintaining a smooth vehicle response to driver demand using the motor while simultaneously closing a clutch that connects the engine to the powertrain (disconnect clutch).
Accordingly, the present invention provides a strategy to start a parallel HEV engine while maintaining a smooth vehicle response to driver demand using the motor while simultaneously closing a clutch that connects the engine to the powertrain. In the preferred embodiment, the HEV powertrain has an engine, a motor/generator, a power transfer unit (such as an automatic transmission, planetary gear set, or an electronic converterless transmission), and an engine disconnect clutch.
The strategy starts running the engine (based on, for example, driver demand), connects the disconnect clutch to the powertrain, fuels the engine, and predicts a desired motor/generator speed. Next the strategy calculates a desired engine torque. And finally, the strategy gradually reduces actual motor/generator torque while proportionally increasing actual engine torque until motor/generator torque is zero while maintaining vehicle velocity.
The prediction of a desired motor/generator speed can be a trajectory comparison based on vehicle velocity and acceleration at a present time and at some past time or on a vehicle accelerator position. Predicting the desired motor/generator speed can also include a determination of whether the vehicle is in speed following control mode.
The system can also add additional strategies such as a termination the strategy if a vehicle brake is applied.
The gradual reduction of actual motor/generator torque occurs by proportionally increasing actual engine torque until motor/generator torque is zero while maintaining vehicle velocity using, for example, a proportional plus integral controller.
Other objects of the present invention will become more apparent to persons having ordinary skill in the art to which the present invention pertains from the following description taken in conjunction with the accompanying figures.